In connection with recent health-conscious trend, there is a growing societal need for measuring an index representing a health condition, i.e., a health index, as an objective numerical value. As to comprehensive biological information, such as body weight, body fat percentage and blood pressure, an apparatus for measuring such information and converting the detected information to electrical information has already been put into practical use. In contrast, an apparatus for directly measuring a numerical value of each biological component has not yet been really put into practical use. In particular, there is no practical apparatus for measuring a concentration of plural types of amino acids in blood or the like, in a simplified manner.
It is known that some amino acids can be used as a health index, and particularly a concentration of branched-chain amino acids (leucine, isoleucine and valine), or a concentration of aromatic amino acids (phenylalanine and tyrosine), reflects a health condition of a liver. Thus, a health index calculated from a concentration of branched-chain amino acids and a concentration of aromatic amino acids is also known as one item of a clinical examination. Specifically, a molar ratio (BCAAs/AAAs) of branched-chain amino acids (BCAAs) to aromatic amino acids (AAAs), so-called “Fischer ratio”, and a molar ratio (BCAAs/Y) of branched-chain amino acids (BCAAs) to tyrosine (Y), so-called “BTR value” as a simpler value than the Fischer ratio, are used as an index representing a liver condition. It is known that a concentration of the branched-chain amino acids (BCAAs) consisting of leucine, valine and isoleucine becomes lower in a cirrhotic patient with advanced liver fibrosis. It is also known that a concentration of the aromatic amino acids (AAAs) consisting of phenylalanine and tyrosine becomes higher in a cirrhotic patient. That is, along with deterioration in liver function, each of the Fischer ratio and the BTR value will become smaller. Thus, each of the Fischer ratio and the BTR value can be used as a health index reflecting a liver condition. A concentration of the branched-chain amino acids is also used as a health index by itself.
While a method using a liquid chromatography process and a sensitivity enhancing technique based on an amino-acid marker in combination is widely used for measuring an amino-acid concentration, the measuring method involves complicated operations, such as preparation of samples, and requires taking a long time for measurement. Further, a biosensor designed to electrochemically detect an enzymatic reaction in an amino acid so as to measure an amino-acid concentration has been increasingly used as a method for quantifying an amino acid simply and quickly even though the applicable amino acid is limited to only several specific types. In most cases, a dehydrogenase is used as the enzyme, and it is necessary to additionally use a coenzyme. As to the conventional measurement of an amino-acid concentration using the biosensor based on an enzymatic reaction involving a dehydrogenase and a coenzyme, there has been known a technique of electrochemically measuring each of amino acids, such as L-leucine, and substances other than amino acids, such as D-glucose, L-lactic acid, ethanol and cholesterol, independently, using a biosensor (see, for example, the following Patent Publication 1). This technique is intended to provide a biosensor capable of electrochemically quantifying a substrate concentration with a high degree of accuracy and within a short time in a simplified manner, and being readily produced at low cost. Specifically, in a biosensor which comprises an electrode system made of an electrically conductive material and formed on an insulating support, and an absorbent carrier containing at least a dehydrogenase, a coenzyme and an electron mediator, as a reaction reagent, and disposed in an electrode reaction region, the absorbent carrier serves as a reaction layer for both an enzymatic reaction between the sample and the reaction reagent and an electrode reaction between the electrode mediator and an electrode surface. Further, as to the biosensor using a dehydrogenase and a coenzyme, there has been known a technique of quantifying each substrate of various samples quickly and in a simplified manner without the need for a complicated pretreatment, by a biosensor using a reaction reagent comprising an electron mediator and a tetrazolium salt (see, for example, the following Patent Publication 2). However, even the above techniques cannot simultaneously measure a concentration of plural types of amino acids.                [Patent Publication 1] Japanese Patent Laid-Open Publication No. 2000-35413        [Patent Publication 2] Pamphlet of PCT Publication WO 00/57166        